My paralells folder is 13GB when I only have about 2 GB of programs and files on my paralells VM. I don't use a boot camp partition just natively installed it on paralells.
Please read the user manual for more details but if you setup your .hdd as expanding then by nature of virtual HDDs will increase in size over time and compressor will bring it back to normal. It's good habit to do this one a month.
What version of Parallels are you using? If you are using version 5160 or newer compression should take about 2 - 7 hours on a 13GB virtual HDD.